Heretofore, polyester films have been found extensive utility in various industrial fields owing to their excellence in mechanical properties, electrical properties, resistance to chemicals, weathering properties, and resistance to heat. Particularly the biaxially stretched polyethylene terephthalate film excels other films in planarity, dimensional stability and modulus of elasticity, and, therefore, it constitutes itself an indispensable materials for printing plates and for substrates in data devices.
The information industries have been remarkably advance in recent years and a number of new devices and articles of various kinds have been developed. Among such devices, there mentioned the electronic white board which have completely changed the concept of the conventional black board. The electronic white board is free from dust of chalk, therefore, is helpful for air cleaning for offices and meeting rooms. Further, an electronic white board provided with a copying device is accepted as an epoch-making article which contributes to the efficiency of meeting. In addition, the advent of the cashless age has brought about increased spread of magnetic cards such as cash card, telephone card, etc.
Biaxially stretched polyethylene terephthalate films provided with white color and high opacifying properties by the addition of a white pigment such as titanium oxide powder are conventionally used as one of the important materials for the above articles.
These films under mentioned are used in the important part of the final product, i.e. used as the copy board part of the electronic white board and as the substrate for magnetic cards due to their excellent planatory and dimentional stability. In these electronic white board and magnetic cards, the films are fully manifesting their outstanding properties mentioned above.
Indeed these films has highly desirable properties. They nevertheless suffer from various drawbacks because they require to contain a white pigment in a very large amount for the purpose of acquiring an enhanced opacifying properties. Since these films contain inorganic particles such as titanium oxide particles having a lagre specific gravity in a ratio exceeding 10% by weight, they have an apparent specific gravity exceeding 1.7 g/cm.sup.3, occasionally exceeding 2.0 g/cm.sup.3, and a weight per unit volume becomes 20 to 50% larger than that of their regular counter-types containing no pigment. This causes heavy labor during the work of slitting, transporting or packing in the course of the production of film products.
In the case of the white board mentioned above, the phenomenon of sagging of the board surface with aging is caused during long-term use, thereby seriously impairing the commercial value of the white board.
Moreover, the large amount of inorganic particles added to the polyester film contribute to increase the rigidity of the film and cause a problem that the film edges are liable to inflict lacerations on operative's hands in the flim slitting process and the cutting process of a product such as magnetic card. Thus, polyester films containing a large amount of such inorganic particles have difficulties in terms of handling properties. The present inventors started a study on a process for producing cellular film from the view point that it is most important for solving the above problems to find out a method for reducing the apparent specific gravity of the film. That is, the reduction of the apparent specific gravity was attempted by providing a film with cellular structure by employing a method in which a cellular product which is used as heat insulating material and cushioning material is produced by incorporating gas (bubble) into thermoplastic resins.
Hitherto, a number of methods has been proposed for producing cellular polyester. For example, a method of incorporation of a gas or a gasifiable substance disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 50-38765(1975), Japanese Pat. Publication No. 57-46456 and Japanese Patent Application Laid-Open (KOKAI) No. 57-34931; a method of incorporation of a substance capable of generating a gas chemical decomposition disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 52-43871 (1977) and Japanese Patent Publication No. 58-50625 (1983); and a method which comprises mixing a material for film with a substance soluble in a solvent, forming the resultant mixture in the form of a film, impregnating the film in the solvent to extract the substance from the film as disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 51-34963(1976) and Japanese Patent Publication No. 52- 27666(1977) can be cited.
However, it was difficult to apply these method directly to the production of biaxially stretched polyester. These methods cited above are those for producing molded articles of certain extent of size. Any of the methods cited above is difficult to be called as a method able to control the cell diameter to a few tens .mu.m or less and to uniform size in consideration of film thickness. Further, these methods can be said to be a method of great difficulty in biaxially stretching an extruded sheet without breaking of the sheet. Mentioned above can be presumed from the fact that these methods have rarely proposed in the specification of patent which relates to biaxially stretched cellular polyester film.
Although, the application of the conventional method for cellular product is very difficult, the formation of minute cell in biaxially stretched polyester film is still effective for reduction of apparent specific gravity.
Further, the formation of a large number of minute cells having a diameter of a few tens .mu.m to a few .mu.m, preferably a diameter corresponds to a wave length of visible light in polyester film can additionally provide the polyester film with a high opacifying properties due to scattering of transmitted light by the cells formed. This, therefore, can promote the reduction of the apparent specific gravity as well as can avoid the rigidity of the polyester film, since the amount of the white pigment can be decreased. Therefore, if it is possible to obtain such a polyester film, the heavy labor during the work of handling can be reduced and can prevent the phenomenon of sagging of film by its weight, so called aging phenomenon.
The present inventors have proposed a process for producing such a cellular polyester film in Japanese Patent Application No. 61-313896(1986), without being swayed by the conventional method for producing cellular products. This process comprises extruding-molding a mixture prepared by adding a specific polypropylene to an aromatic polyester into a sheet, stretching the sheet at least in one direction, thereby obtaining a polyester film containing minute cells in the surface and inner parts thereof. Thus, the reduction of the apparent specific gravity has been achieved by forming minute cells in films by the method quite different from the conventional methods. Additionally, this method is easily reduced to practical use without imparing the productivity, therefore, the invention providing such method is epoch-making.
However, although the method is epochal in forming minute cells in films, it is not necessarily sufficient in opacifying properties of films and the films obtained by the method are obliged to greatly decrease its mechanical strength. In the circumstances, it has been desired to develop a method for making cells formed in the surface and inner parts of film into minute cells in order to provide the film with more high opacifying power.